In working with high pressure fuel injectors wherein the pressure of the fuel exceeds 6.5 Bar, typically over 65 Bar, the materials used in the construction of the major parts of the injector can develop pressure cracks over life which will allow fuel to seep out externally. As can be appreciated the leakage of fuel from an injector will result in very unfavorable fuel economy.
In solenoid operated injectors, the terminals of the solenoid coil are typically brought out of the injector through an end cap. The terminals may be suitable for connection to a connector or may be suitable to receive wire terminals. In either event, the terminals must be insulated from the main body of the injector. This is so since most injectors are fabricated from a metal housing and are located in the engine block or manifold.
With the requirement for insulating the terminals from the housing, many end caps are molded from a plastic material. In low pressure injectors, such plastic materials, either thermoset or thermoplastic materials, solve both the insulating problem and they also are solid enough to be leak proof.
In earlier high pressure injectors, the end caps were likewise fabricated from various thermoset and thermoplastic materials. While these solved the insulating problems, over time, very small cracks developed in the caps through which fuel seeped.